let str = '1safb'
let length = str.length
let new_str = ''
let object = {
  name: 'mike',
  age: '19',
}
arr = [1, 2, 3, 5 ,8 ,6 ,0 ,9 ]
for (let i in object) {
  console.log(i);
}

// for (let j of object) {
//   console.log(j);
// }

// for of 不能遍历Object  
//forof:str arr  
//forin:str arr Object